I would like to block all attachments (Outgoing only) on mail server and allow only sending to some domains which I allow specifically.

We have approx 15 domains and out of 15 three domains are in iRedMail server, rests are on Google. Many users are sending sensitive attachments to their personal E-mail IDs which we want to restrict and only allow sending attachments to all our 15 domains. However users will not be restricted with attachment when they receive from anywhere on internet.

I think we can do this via amavis but I do not how to do it, your kind help is highly appreciated.